Laboratory-accelerated weathering tests

Light Sources in Laboratory-Accelerated Test Devices 

HMI metal halide lamp (—), and daily average of daylight in Miami, FL measured at normal incidence during the vernal eqninox nnder clear sky conditions (-—). (Courtesy of Atlas Material Testing Technology LLC.) 

Both types of carbon arc devices have capabilities for periodic water spray on the samples and condensation during a dark period as weU as for humidity and temperature control. Irradiance is not monitored or electrically controlled as in xenon arc devices, bnt exposure is maintained constant by the requirement of daily replacement of the carbon rods. Carbon arc exposures, particularly using the enclosed carbon arc, are largely being replaced with xenon arc tests. 

The fluorescent UV sources are typically incorporated into fluorescent UV/condensation devices in which dark periods with condensation are alternated with periods of UV radiation. The temperature is generally controlled at different levels during each of these periods. The devices may also incorporate periodic water spray on the front surface of the samples during exposure to radiation but do not control relative humidity. Some of the modem types of these devices have capabilities for irradiance control. Procedures for exposing polymeric materials in 

Since the effect of temperature varies with type of polymer aud its formulations, temperatures different than those encountered in end-use environments can distort the stability rankings of materials in addition to causing unrealistic aging behavior. For example, it was shown [141] that change in air temperature from 30 to 60°C in an artilicial weathering test changed the rank order of the stabilities of polyamide, polypropylene, and polyester yam based on reduction 

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The light sources commonly used for laboratory-accelerated weathering tests include filtered xenon arcs, two types of filtered carbon arcs, fluorescent UV lamps, and metal halide lamps. The spectral power distributions are shown in Figures 8.3 through 8.6 in comparison with solar radiation. The spectral emission characteristics of the sources differ significantly in both the UV and visible regions. 

Laboratory accelerated weathering tests have played an important role in development of polymeric materials with highly improved weatherability. However, since all stresses present in an outdoor exposure cannot be simulated in a laboratory accelerated test, the latter cannot replace natural exposure. It is a complimentary technique, the usefulness of which largely depends on how closely it reproduces the chemistry and weathering effects caused by the slower outdoor exposures. Thus, correlation is a fundamental issue which must be considered when selecting a laboratory accelerated weathering method. It depends on reasonably representing... 

Laboratory accelerated weathering devices have been used for more than 80 years with increasing importance concomitant with the development of more weather-able materials and the need to determine in a short time the effects of natural exposures over prolonged periods. The importance of these devices lies in their ability to accelerate the weathering processes imder controlled and reproducible conditions. They are particularly useful in research and development of new polymeric formulations. They are also used for quality control and specification testing. However, their application to prediction of service life under use conditions is still under development (see section on Laboratory Accelerated Versus Natural Weathering). 

The main goal of laboratory-accelerated weathering experiments is often to predict outdoor weathering performance. Many variables have to be well understood and properly applied in order for the simulated tests to be successful. The most critical variable is the light and the effect of the spectral distribution of light both outdoors and in artificial weathering, although this remains an imperfectly solved problem. 

In order to provide a reliable early evaluation of the weatherability of a polymeric material, ideally the laboratory accelerated test should satisfy all correlation criteria when compared with results of natural weathering. However, any accelerated weathering test, either artificial or natural, only approximates the field exposure conditions. Nevertheless, the closer the simulation of the natural balance of all important weathering factors by the accelerated test, the better the... 

In the laboratory, the degrading agencies are usually separated. However, in some accelerated weathering tests, a high-intensity mercnry or xenon arc, water spray, and temperature and hnmidity control may be used. Outdoor exposure itself... 

On the basis of this evidence, it is recommended that an experimental test program, consisting of both laboratory and field tests, be developed and Implemented to quantitatively measure the effects of acid deposition on both the as thetic and structural properties of PCC structures. It is, however, recommended that a preliminary series of controlled, accelerated laboratory tests be carried out before a full-scale field evaluation program is instituted. The objectives of the accelerated laboratory test program should be to identify the magnitude of the problem and to attempt to differentiate between the effects of wet deposition, dry deposition, and normal weathering. The preliminary test program should concentrate on surface chemistry effects and penetration rates of SO4, NOx, Cl as deposited from wet and dry deposition. The... 

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